(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly to an improved method and procedure of plasma chamber cleaning procedures.
(2) Description of the Prior Art
Due to its superior performance over wet etching, plasma dry etching has been broadly applied in present semiconductor manufacturing to form active areas, lines, holes and other features created for the construction of the devices. During plasma dry etching, processing gases are introduced into an etching chamber where they are excited by radiation frequency (rf) electromagnetic waves to form reactive etchants. The etchants react with the unmasked substrate areas and form volatile species departing from the substrate thereby forming desired features of a semiconductor device. The plasma becomes a complex mixture of native etching gases and etched products, including molecules, ions, neutral radicals and other species.
Some by-products can deposit on the insides of the chamber walls and cause serious problems as they accumulate to a particular density. For example, in addition to affecting the chamber processing conditions thereby degrading the processing stability, the accumulated by-products on the chamber walls may dislodge (from the chamber walls) and settle down on the surface of the substrate causing fatal defects that are likely to negatively affect the entire chip. Therefore, the removal of the accumulated by-products becomes extremely important. In-situ plasma dry cleaning is desirable due to its efficiency. During plasma dry cleaning, cleaning gases are introduced into the chamber. The cleaning gasses interact with the by-products (of the process taking place in the chamber) and form volatile species that are pumped out-off the chamber thereby realizing a "cleaning" of this chamber. With this technique, it is not necessary to open the chamber for cleaning; thus no time-consuming machine-setup is required after cleaning. However, the highly reactive cleaning gasses may also attack the chamber wall and shorten its lifetime. Currently, time-mode is typically used for plasma dry cleaning. In this time-mode the cleaning process is performed for a fixed period of time to remove the by-products accumulated on the chamber walls. It is obvious that either under-cleaning or over-cleaning is very likely to occur. The former causes insufficient by-product removal while the latter results in erosion of the chamber wall. In addition, non-volatile aluminum fluoride may be formed as the cleaning plasma attacks the chamber wall during over-cleaning, resulting in an aluminum fluoride particle problem. Therefore, the most desirable approach becomes an endpoint mode that does not result in significant erosion of the chamber walls by the plasma that is formed in the chamber during the process of cleaning. This method of endpoint detection must however by very reliable. The essence of the invention is to provide a method for endpoint-controlled plasma dry cleaning that is based on the optical emission spectrum of the plasma that is formed inside the chamber during the process of cleaning the chamber. Optical emission has previously been utilized in plasma dry etching for endpoint detection. Each kind of molecule or atom has its characteristic optical emission at specific wavelengths. While an overlying layer is being removed, the underlying layer may become exposed to the etching plasma. Thus, different etching by-products are expected to enter the plasma (within the chamber) and accordingly cause a change in the plasma optical emission spectrum. This change in the plasma optical emission spectrum is used to detect the etching endpoint. The objective of the invention is to introduce the principle of endpoint-controlled processing into plasma dry cleaning. When the cleaning plasma etches away the accumulated by-products, related product species may be formed. These related product species can be identified by their unique and characteristic optical emission. After the accumulated by-products have been removed from the chamber and the chamber walls are exposed to the plasma, there is a change in the optical emission. This change in optical emission can be used to detect the endpoint of the cleaning process and to therefore stop the cleaning process.
An in-depth grasp of the cleaning process depends on an understanding of the molecule/atom excitation and decay transition mechanism. In a molecule or atom, electrons are present in various states of energy distribution in which the individual electrons have their specific energies. As an electron transits (decays) from a higher energy state to a state of lower energy, the electron emits a photon that possesses energy equal to the difference between the two energies of the two orbits of the electron. Because a given molecule/atom has a specific energy state structure, that is a specific energy level structure, the energy of the emitted photon, and therefore its wavelength, is characteristic of the molecule/atom. In other words, each kind of molecule/atom corresponds to its characteristic optical emission spectrum by which the molecule/atom is theoretically identifiable. In plasma, the various species of the plasma have (their characteristic) electrons that, when the electrons decay from a higher energy level to a lower energy level, are excited and as a consequence emit their characteristic spectra. As the plasma etching has removed an overlying layer and penetrates to the underlying material, different products are generated; thus a change in the emission spectra takes place. This change is used to signal the endpoint.
Molecular theory teaches that, for actual molecules/atoms combinations, one state of the molecules/atoms combination usually does not correspond to a single energy level, but to a series of adjacent sub-energy levels as shown in FIG. 1. The reason is that the single energy level (corresponding to a particular state) splits into multi-levels due to such factors as vibration and angular momentum coupling. Electrons decay from a higher level of energy to a lower level and, as a consequence, emit photons during this decaying process. Each energy level as shown in FIG. 1 consists of a series of sub-energy levels due to the "energy-level splitting".
This variety of factors contributes to the highly irregular pattern of energy or emission intensity as a function of wavelength that is observed in a typical emission spectra, FIG. 2. The emission intensity has been plotted along the Y-axis while the wavelength (in nm) has been plotted along the X-axis. The emission spectrum that is shown in FIG. 2 derives its characteristics from the material of which the emitting source, in this case the plasma in the chamber, is made and from the way in which the material is exited. The sub-peaks shown in FIG. 2 result from the above indicated "energy-level splitting" effect. Specifically shown in FIG. 2 is the electromagnetic energy generated by Si--F, which has a wavelength of optical emission within the range between about 430 and 460 nm.
U.S. Pat. No. 5,465,154 (Levy) teaches a method of monitoring the etch rate of materials using a light beam.
U.S. Pat. No. 5,468,686 (Kawamoto) shows a method of cleaning an etch chamber.
U.S. Pat. No. 5,712,702 (McGhahay et al.) shows a method to determine the chamber clean end point by sensing the exhaust.
U.S. Pat. No. 5,811,356 (Murugesh et al.) shows a chamber seasoning method.
U.S. Pat. No. 5,824,375 (Gupta) shows a decontamination of a chamber after plasma clean.